1. Technical Field
The present invention relates to wireless communications and, more particularly, wideband wireless communication systems.
2. Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards, including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, etc., communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of a plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via a public switch telephone network (PSTN), via the Internet, and/or via some other wide area network.
Each wireless communication device includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier stage. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier stage amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives an inbound RF signal via the antenna and amplifies it. The one or more intermediate frequency stages mix the amplified RF signal with one or more local oscillations to convert the amplified RF signal into a baseband signal or an intermediate frequency (IF) signal. As used herein, the term “low IF” refers to both baseband and intermediate frequency signals. A filtering stage filters the low IF signals to attenuate unwanted out of band signals to produce a filtered signal. The data recovery stage demodulates the filtered signal to recover the raw data in accordance with the particular wireless communication standard.
The demand for high performance local oscillator generators (LO-GENs) for use in RF transceivers is growing with the increasing performance and integration requirements of wireless communications systems, such as WiFi systems and cellular telephony. The oscillators used in RF transceivers are usually embedded in a frequency synthesizer environment, so as to achieve a precise definition of the output frequency.
Synthesizer design remains a challenging aspect of RF system design, because of the stringent requirements typically imposed on frequency synthesizers. For example, RF transceiver LO-GENs are typically required to be defined with an output frequency accuracy on the order of a few parts per million (PPM). Furthermore, in most cases, the output frequency must also be capable of being varied in small precise steps, such as a few hundred kilo-hertz (kHz), corresponding to the RF channel spacing.
In addition to accuracy and channel spacing, several other aspects of LO-GENs influence the performance of a transceiver, such as phase noise, reference spurs and lock time. The phase noise of the LO-GEN impacts both the receive and transmit paths. For the receive path, if the phase noise mixes with nearby interferers that are then converted onto the desired channel, the signal-to-noise ratio of the received signal can be adversely affected. In addition, reference spurs may cause the receiver to down-convert undesired interferers, and may cause the transmitter to violate spectral mask requirements specified by the communications standard. Furthermore, the lock time required in typical RF systems varies from a few milliseconds (ms.) to a few tens of microseconds (us.). As used herein, the term “lock time” refers to an indication of how fast a new frequency is established when the RF transceiver commands a change in the channel.
Two types of frequency synthesizers based upon phase locked loops have gained wide spread use in wireless systems: the fractional-N and the integer-N frequency synthesizers. Fractional-N frequency synthesis is a popular indirect frequency synthesis method for high performance applications, such as cellular telephony, due to the ability of fractional-N synthesizers to synthesize frequencies over wide bandwidths with narrow channel spacing. For example, in GSM cellular telephony, one pair of RF bands, i.e., transmit (TX) and receive (RX) bands, consists of the frequencies 880.2 MHz -914.8 MHz and 925.3 Mhz-959.9 MHz, respectively. Within each of these bands, the GSM channel spacing is 200 kHz. The fractional-N frequency synthesizer is capable of achieving a very fine output frequency resolution to accommodate such narrow channel spacing.
A fractional-N PLL frequency synthesizer typically includes a precise crystal oscillator (X-TAL) providing a reference frequency, a phase and frequency detector (PFD), a charge pump (CP), a lowpass loop filter (LPF), a voltage controlled oscillator (VCO), and several divider blocks in the feedback path that each divide the incoming signal by some integer of either fixed or on-the-fly programmable value. Typically, the fixed dividers are in the front-end of the divider chain, while the programmable divider (referred to herein as the “multi-modulus divider”) is the last divider stage before the feedback signal is inputted to the PFD feedback terminal to achieve fine (e.g., fractional) frequency tuning of the output signal.
However, fractional-N frequency synthesizers suffer from a critical drawback, namely the generation of in-band spurs due to non-linear behavior of the PFD and charge pump. Ideally, the charge transferred into the loop filter is proportional to the phase difference between reference and feedback signals. In practice, however, the characteristics of the PFD/CP combination does not provide a completely linear transfer curve. When the frequency synthesizer is operating in lock, the phase difference assumes different values due to the changing divide ratio in the feedback path. Thus, the changing divide ratio triggers non-linearities of the PFD/CP, and creates a noise floor that increases the in-band phase noise of the frequency synthesizer. Such increase of phase noise may be prohibitively large for systems with stringent phase noise requirements.
A frequency synthesizer approach that can be designed to satisfy very stringent phase noise requirements is the integer-N frequency synthesizer. The integer-N frequency synthesizer includes the same components as the fractional-N synthesizer with the difference being that the multi-modulus divider does not change dynamically during operation of the synthesizer in a particular RF channel. In other words, the divide ratio of the feedback path remains constant for operation in-between channel steps. The fact that the divide ratio is kept constant causes the PFD/CP combination to operate highly linearly since the phase excursions of the feedback signal are very limited.
However, the integer-N architecture requires that the reference frequency be equal to the desired channel spacing due to the fact that the resolution of the feedback divider is one. Thus, for a channel spacing of 200 kHz, the PLL reference signal must be 200 kHz. Such low reference signal gives rise to considerable “reference spurs” (i.e., periodic modulations of the VCO generating tones around the RF carrier).
For example, when the loop is completely in lock, ideally, no pulses are generated by the CP. In practice, due to finite reset delay of the PFD and due to current source mismatches of the CP, such ideal equilibrium is not achieved. Rather, the VCO control voltage experiences a finite transient at each phase comparison instant around an equilibrium point. Thus, the VCO output, i.e., the RF carrier, contains sidebands corresponding to such “feed-through” of the reference frequency. The problem of reference spurs is a difficult one, especially if the reference spurs fall in-band. Designing the PLL with a narrow signal filter may provide some attenuation of such reference spurs, but in many systems it is difficult or impossible to attenuate such spurs enough while at the same time employing a signal filter bandwidth large enough to ensure fast synthesizer settling (lock time).
Thus, fractional-N and integer-N frequency synthesizers each have advantages and drawbacks. For fractional-N synthesizers, the advantages are fine frequency resolution, while employing a relatively high reference frequency. This allows for a strong attenuation of the reference spurs through the PLL signal filter while still maintaining wide enough bandwidth for fast synthesizer settling (lock time). The drawback of the fractional-N synthesizer is an elevated in-band phase noise level due to the triggering of PFD/CP non-linearities. For integer-N synthesizers, the advantage is generally very low in-band phase noise. The drawback is a high reference spur level, which can only be reduced by narrowing the PLL signal filter. Such narrowness results in slow synthesizer settling (lock time).
Therefore, a need exists for a frequency synthesizer design for use in transceivers that combines the advantages of the fractional-N and integer-N approaches, but without the mentioned drawbacks.